K. Chang has isolated MAb K1 which reacts with many non mucinous ovarian cancers, mesotheliomas and some squamous cell carcinomas. The antigen with which MAb K1 reacts is a 40,000 molecular weight glycoprotein containing about 25% carbohydrate and is attached to the cell membrane through a phosphotidylinositol linkage. The antigen is present in normal mesothelium as well as many cancers. Recently, cDNAs encoding the K1 antigen have been isolated and are currently being sequenced. The K1 antigen is poorly internalized so that K1-PE40 is not very cytotoxic to target cells. The cDNAs encoding the light and heavy chains of K1 were isolated and a single chain immunotoxin constructed. In addition, Fab fragments have been made in E. coli and will be used for imaging studies.